Q79 of 84 Page 5

If the sum of first n terms of an A.P. is (3n2 + 7n), then find its nth term. Hence write its 20th term. (CBSE 2015)

Sn = (3n2 + 7n)

First term, a = S1 = [3(1)2 + 7(1)]


= [3 + 7] = 5


S2 = [3(2)2 + 7(2)]


a + a + d = [12 + 14]


5 + 5 + d = 13


d = 3


an = a + (n – 1)d


= 5 + (n – 1) 3


= 5 + 3n – 3


= 3n + 2


a20 = 3(20) + 2


= 60 + 2 = 62


Hence, it’s 20th term is 62

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